Establishment of networks that can freely be controlled, which is referred to as software defined networks (SDN) has been proposed. As a typical example of such SDNs, NPL (Non-Patent Literature) 1 proposes a communication protocol referred to as OpenFlow and basic operations of a packet forwarding apparatus. In OpenFlow, an external communication control apparatus called an OpenFlow controller is arranged, and a packet forwarding rule can externally be set in a packet forwarding apparatus. In this way, the external communication control apparatus can manage communication forwarding control, and a new communication control system can easily be realized through development of an external communication control apparatus.
However, these SDNs have a problem that external communication control apparatuses have load for control determination. Thus, if only one external communication control apparatus is used, it is difficult to manage packet forwarding apparatuses in a centralized manner. Therefore, it is desirable that the load for control determination be distributed. NPL 2 describes a solution to this problem. NPL 2 describes a hop-by-hop system and an overlay system separately. In the hop-by-hop system, an external central manager manages all paths to perform communication management for optimization and adjustment for each purpose. The hop-by-hop system has problems with storage of information about all paths as described and concentration of other packet forwarding setting operations, for example. Thus, a solution to the scalability problem is demanded. In the overlay system, a controller is arranged for each managed group, and a setting between managed groups is deemed and performed as a communication therebetween. In addition, by applying a tunneling technique or the like to a communication between managed groups and managing the communication as a single logical network, the communication between managed groups can also be realized consistently. The overlay system mitigates the scalability problem. However, since the managed targets are abstracted as being nodes in a group, between groups, and the like, it is easily to assume that the same problem will arise as the number of managed targets increases.
To solve the above scalability problem, various systems have been proposed and implemented. NPL 3 describes a technique as one typical example of such systems.
NPL 3 describes an example of the architecture of a communication control system using OpenFlow. Processing components necessary for control are defined and processing of each of the components is also defined. In NPL 3, consideration is given to how information is held for scalability. According to NPL 3, an all-node duplication mechanism and a distributed storage mechanism as typified by a DHT (Distributed Hash Table) are stored among nodes. Information that is important and updated less frequently such as information about switch and link configurations is stored in the all-node duplication mechanism and information updated more frequently such as information about link usage levels is stored in the distributed storage mechanism.    NPL 1    “OpenFlow Switch Specification”, Version 1.1.0 Implemented (Wire Protocol 0x02), [online], Feb. 28, 2011, [searched on Feb. 14, 2012], Internet <URL:http://www.openflow.org/documents/openflow-spec-v1.1.0.pdf>.    NPL 2    “Feature Article, Evolving Network Virtualization, Software Defined Network OpenFlow and OpenStack”, Nikkei Communication, 2011 Nov. 1 issue, pp. 14 to 17.    NPL 3    “Onix: A Distributed Control Platform for Large-scale Production Networks”, [online], USENIX OSDI 2010, Retrieved 2010-10-01, [searched on Feb. 14, 2012], Internet <URL:http://www.usenix.org/event/osdi10/tech/full_papers/Koponen.pdf>.